The present invention relates to a method for photoelectrically detecting the presence of bubbles and/or inclusions in an optical fiber preform as well as an apparatus for detecting these bubbles and/or inclusions.
Bubbles and inclusions present in an optical fiber become causes of fatal defects, since they scatter and/or absorb light rays which transmit through the optical fiber to thus attenuate the intensity of the transmitting light rays and reduce the strength of the optical fiber. When drawing an optical fiber preform into an optical fiber, the optical fiber may sometimes be broken due to the presence of these bubbles and inclusions. It is thus needed to inspect an optical fiber preform for the presence of bubbles and inclusions prior to drawing it into an optical fiber in order to eliminate the foregoing problems. An optical fiber preform has conventionally been inspected for the presence of bubbles and/or inclusions by making light rays incident upon the preform and visually observing the preform, with the naked eyes, for the presence of bright spots formed by the bubbles and/or inclusions present therein which scatter the light rays incident upon the preform. This is one of sensory tests and therefore, there are differences between individuals in the detection of such bright spots. Moreover, there has been desired for the establishment of a method for mechanically inspecting optical fiber preforms, for the elimination of the foregoing problem and for the elimination or reduction of labor.
A method for inspecting the influence of bubbles present in a quartz tube used as a jacket for an optical fiber on various properties of the resulting optical fiber was reported at the Annual Meeting, the 59th year of Showa, of Electronics Telecommunication Society of Japan, A National Convention of the Branch of Optics Radio Wave. The gist thereof is disclosed in the article entitled "Effects of Bubbles Present in Quartz Tubes on the Strength of Optical Fibers", the published Resume for the Meeting, 2-165, 421. According to this article, the method comprises making a laser beam incident upon the end face of a quartz tube while rotating the quartz tube and detecting light rays scattered and reflected by bubbles present in the tube with a photomultiplier. The quartz tube is irradiated with the laser beam along the circumference of the tube as the tube is rotated and the quartz tube can be inspected by scanning the photomultiplier along the longitudinal direction of the tube.
However, this method is inconvenient for the detection of bubbles throughout the whole length of the quartz tube. This is because, the cross sectional area of the laser beam is substantially smaller than the thickness of the quartz tube and accordingly, it is impossible to irradiate the whole cross section of the tube with the laser beam. When inspecting, in particular, a subject having a circular cross section such as an optical fiber preform, only a part thereof can be inspected by irradiating the cross section thereof with a laser beam. For this reason, there has not yet been proposed any method for mechanically detecting bubbles and/or inclusions present in an optical fiber preform.